For offices decided by RCV, voters rank their first, second and third choices. Through a complicated system of algorithms, if no candidate gets 50 percent-plus-one after counting all the first-place votes, the last-place candidates are eliminated one by one, and their voters' second-choice votes are redistributed.

The first time it was used in a San Francisco election was 2004. It didn't go well.

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"We went to push the button and we had a lot of media attention and all the campaigns were there -- and it didn't work," San Francisco Elections Director John Arntz recalled. The winners were eventually figured out.

After that disaster, the city modified the voting machine software used to allocate and count second- and third-place votes. The new "improved" system was rolled out in 2008. It didn't work either.

Now there's more trust that the RCV system works the way it's supposed to, said Arntz, adding that "it's not as much of a mystery to people."

Still, in describing recently how RCV votes are counted and reported out to the public, Arntz got a little tongue-twisted and soon acknowledged how complicated it was to explain.

"Wow -- try writing a memo about this stuff, man!" he said.

Proponents of RCV say it avoids costly runoff elections with low turnout and encourages more civilized campaigning as candidates avoid alienating their opponents' supporters in hopes of winning their second-place vote.

Political consultant Eric Jaye, who's running Supervisor Jane Kim's campaign for mayor, wouldn't say whether he likes RCV or not, but he acknowledges that it deprives voters of a one-on-one comparison that existed under the old system.

"It removes that intense runoff where the candidates get vetted and tested by a campaign," Jaye said, adding that "we’re running under the system we have. It’s not the time to criticize it."

Jaye's candidate Kim recently teamed up with former state Sen. Mark Leno, the other leading progressive candidate for mayor, creating a video ad with each saying they support the other as their second-place vote.

With most polls showing Board of Supervisors President London Breed in first place, collecting a large number of second-place votes may be the only way for Kim or Leno to finish first.

It's a strategy that was used in the Oakland mayor's race in 2010.

City Supervisors Jean Quan and Rebecca Kaplan teamed up against the favorite -- former state Sen. Don Perata -- with an "anybody but Don" message. While Perata was on top after all the first-place votes were counted, Quan edged him out 51 to 49 percent after second- and third-choice votes were added.

Perata's political consultant at the time, John Whitehurst, called it "a travesty," noting that his candidate won 78 percent of the city's precincts but ended up losing. He called ranked-choice voting "an injustice."

Today, Whitehurst -- who is not helping any candidate in the San Francisco mayoral election -- says RCV tends to create candidates with broader appeal and fewer sharp differences.

"I believe that ranked choice voting still does not provide voters with a good, clear choice at the end of an election cycle," Whitehurst says.

In an effort to win second-place votes, he says the candidates "want to be all things to all people," adding "there are very few who can explain how the tabulation of ranked choice voting works."

The name "instant runoff" is a bit of a misnomer.

San Francisco elections chief Arntz says it could be the weekend after the June 5 primary before a winner of the ranked choice voting contest is known.